The invention claimed and disclosed herein pertains to Micro Electro Mechanical Systems (xe2x80x9cMEMSxe2x80x9d, MicroElectroMechanicalSystems or micro-electro-mechanical systems), and in particular to MEMS having stiffened beams (sometimes called flexures), and methods for stiffening beams in MEMS.
The present invention is directed towards structures which are used to support suspended masses in MicroElectroMechanicalSystems (xe2x80x9cMEMSxe2x80x9d). MEMS, as the name suggests, are microelectromechanical systems that provide mechanical components and electrical components (collectively, microelectromechanical components, which make up microelectromechanical devices, or xe2x80x9cMEMDsxe2x80x9d) to produce a micromachine capable of controlling, or responding to, an environmental condition within the system. The mechanical components and electrical components in MEMS are typically dimensionally measured in microns (1xc3x9710xe2x88x926 meters). MEMS devices typically include microsensors to detect changes in the system""s environment, an intelligent component (such as a control logic circuit) which makes decisions based on changes detected by the microsensors, and microactuators which the system uses to change its environment. Example MEMS devices include inkjet-printer cartridges, accelerometers that deploy airbags in automobiles, and inertial guidance systems. MEMS are typically fabricated from, or are fabricate on, a substrate, and are fabricated using known (as well as newly developed) technologies. One of the primary technologies used in the fabrication of MEMS are depositional and mask technologies which are applied to fabricate the MEMS much in the same manner that semiconductor devices such as microprocessors and memory devices are fabricated. MEMS fabrication techniques also include photoetching and/or micromachining to remove part of one or more of deposited layers to thus define one or more of the mechanical or electrical devices. Micromachining often is performed using a focused ion beam such as an eximer laser for small adjustments to prototypes.
Certain micromechanical and microelectromechanical devices (xe2x80x9cMEMDsxe2x80x9d) comprise masses or mass elements which define surfaces which are supported by a beam or a bridge. The beam or bridge acts as a flexible member allowing the mass element to move with respect to surrounding structures in the MEMS. Typically the mass element is supported by the beam or bridge such that it is free of contact with the surrounding elements of the MEMD. The mass element can be part of a microactuator or a microsensor, as well as a component in other types of MEMDs. For example, a microactuator can be configured to drive a resonant sensor to cause the sensor to oscillate at its resonant frequency. In other applications microactuators can be used to produce a mechanical output required for a particular microsystem. An example of this latter application is using a microactuator to move micromirrors to scan laser beams (as for example in laser printing). Accordingly, the mass element which is supported in a MEMS by a bridge or beam defines an xe2x80x9carea of interestxe2x80x9d which can support microsensor components or other components which comprise parts of a microactuator. Alternately, the mass element itself can have electrical properties, such as conductance, capacitance or resistance, and the mass element can thus itself be used as the active component in the MEMD.
In general, it is desirable that the beam or bridge used to support a mass element in a MEMS have sufficient structural strength that it can support the mass element above another surface, or between two other surfaces. Turning to FIG. 1, a plan view of a section of a MEMS 10 is depicted in a simplified sectional plan view. The section of the MEMS depicts a component of a microelectromechanical device 11, which comprises a mass element 12 supported by a cantilevered beam section 20. The MEMD component 11 is defined from the surrounding substrate 16. This can be accomplished using photolithographic processes, for example, wherein the zone 18 between the device 11 and the rest of the substrate 16 is removed by etching or micromachining. The mass element 12 defines an area of interest 14. FIG. 1 (and the other figures discussed below) is not to scale, but is depicted so as to facilitate understanding of the prior art. The prior art beam section 20 is a solid beam from its first end 21 where it is connected to the substrate 16, to the point of attachment 23 to the mass element 12. The beam section of these MEMD components must be sufficiently rigid such that movement of the mass element 12 does not occur in a direction indicated by the arrow xe2x80x9cXxe2x80x9d, but rather occurs in a direction into and out of the plane of the sheet on which the figure is drawn. (A brief review of FIG. 3, which depicts a similar prior art device, shows that the mass element 12 is intended to move in the directions indicated by the arrow xe2x80x9cYxe2x80x9d). If the beam section 20 (FIG. 1) is insufficiently stiff to resist bending on the beam in the xe2x80x9cXxe2x80x9d direction (FIG. 3), then the mass element 12 can come into contact with the inner wall 28 (FIG. 1) of the substrate 16. This can cause improper sensor readings, failure to actuate another device, or complete failure of the device 11 should the mass element 12 become stuck due to friction between the mass element 12 and the sidewall 28. One method to provide sufficient rigidity in the xe2x80x9cXxe2x80x9d direction is to make the beam quite wide (i.e., a relatively large dimension in the xe2x80x9cXxe2x80x9d direction).
However, the beam section 20 must be sufficiently flexible to allow the mass element 12 to oscillate relatively freely into and out of the plane (xe2x80x9cYxe2x80x9d of FIG. 3). However, these two objectivesxe2x80x94allowing flexibility in the xe2x80x9cYxe2x80x9d direction while providing rigidity in the xe2x80x9cXxe2x80x9d directionxe2x80x94are at cross-purposes. One problem with increasing the width of the beam section 20 to provide rigidity in the xe2x80x9cXxe2x80x9d direction is that it increases the mass of the beam, thus requiring more power to cause the mass element 12 to move. Further, providing a massive beam requires that complex calculations be performed to calibrate a microsensor or a microactuator in which the device 11 is a component. If the mass of the beam 20 were sufficiently small, then it""s mass could potentially be ignored when making these calculations, greatly simplifying the design task.
One prior art solution to this problem is depicted in FIG. 2. FIG. 2 depicts a partial plan view of a slightly modified micromechanical device 11xe2x80x2 in a MEMS 10xe2x80x2. The device 11xe2x80x2 is in most respects same as the device 11 depicted in FIG. 1. Like-numbered components between the devices 11 and 11xe2x80x2 of respective FIGS. 1 and 2 are essentially the same. However, the micromechanical device 11xe2x80x2 of FIG. 2 has a modified beam section 20xe2x80x2. The beam section 20xe2x80x2 comprises a first beam member 24, and a second essentially parallel beam member 26, thereby defining an opening 22 between the two beam members. Each beam member is defined by a thickness xe2x80x9cTxe2x80x9d. The micromechanical device 11xe2x80x2 is further depicted in a side elevation sectional view in FIG. 4. FIG. 4 is provided to facilitate understanding of how the device 11xe2x80x2 can operate, and depicts an example wherein the device 11xe2x80x2 is a microsensor. The mass element 12 of the device 11xe2x80x2 supports a material (such as an electromagnetic material) 32 and 34 on the respective upper and lower surfaces of the mass element 12. Semiconductor material 36 and 38 is formed into the surrounding substrate 16 proximate to the respective upper and lower surface of the device mass element 12. As the mass element 12 is oscillated in the xe2x80x9cYxe2x80x9d direction, an electrical property (such as electrical current or voltage) can be induced or varied between the semiconductor surfaces 36 and 38. This variance can be detected and measured. As an environmental condition (such as temperature) affects the device 11xe2x80x2, the rate of oscillation of the mass element 12 will be varied, which can be detected by the semiconductors 32 and 34 since the measured electrical property will change as a result of the change in the rate of oscillation of the mass element 12. Turning to FIG. 3, an isometric sectional view of the prior art MEMS 10xe2x80x2 of FIG. 2 is depicted. As can be seen, each beam member 24 and 26 of the beam section 20xe2x80x2 is further defined by a height xe2x80x9cHxe2x80x9d.
The prior art design depicted in FIGS. 2-4 reduces the mass of the beam section 20xe2x80x2 over that of the prior art beam 20 depicted in FIG. 1, and also provides more flexibility in the xe2x80x9cYxe2x80x9d direction for the mass element 12 (see FIG. 3). However, in order to prevent movement of the mass element 12 in the xe2x80x9cXxe2x80x9d direction, the beam members 24 and 26 must be relatively thick (xe2x80x9cTxe2x80x9d of FIG. 2). Also, the design depicted in FIGS. 2 and 3 reduces the resistance to torsional bending over the design depicted in FIG. 1 so that it becomes easier for the mass element 12 to rotate in the direction indicated by the arrow xe2x80x9cTxe2x80x9d about axis xe2x80x9cZxe2x80x9d. Such torsional bending can lead to erroneous results from the micromechanical device 11xe2x80x2, and can also lead to the mass element 12 becoming wedged against the inner wall 28 of the substrate 16 (see FIG. 1). Torsional bending can be reduced by increasing the thickness xe2x80x9cTxe2x80x9d (FIG. 2) of the beam members, or by increasing their height xe2x80x9cHxe2x80x9d (FIG. 3), but both of these solutions work against the goals of reducing mass of the beam and increasing the flexibility of the beam in the xe2x80x9cYxe2x80x9d direction.
A second type of micromechanical device is depicted in FIGS. 5A and 5B. FIG. 5A depicts a side elevation sectional view of a microactuator 40 which is defined in a surrounding substrate 48. The device 40 is known as a comb drive and comprises a mass element 42 which is supported in the substrate 16 by two bridge members 41 and 43. The mass element 42 supports an upper series of fingers 45, and a lower series of fingers 44. Fingers 44 and 45 are interdigitated with respective static fingers 47 and 46, which are supported by the substrate 48. The bridge members 41 and 43 are designed to be flexible to allow the mass element 42, and thus the supported series of fingers 44 and 45, to move in a direction xe2x80x9cAxe2x80x9d as depicted in FIG. 5B when a voltage is applied between fingers 46 and the mass element 42. Likewise, when a voltage is applied between the mass element 42 and fingers 47, the mass element will move in the opposite direction. The movement of the mass element 42 in response to an applied voltage is proportional to the number of fingers as well as the bending resistance of the beam members 41 and 43. Since it is generally desirable to use as little power as possible to actuate micromechanical devices, the device 40 can be made more responsive by either increasing the number of fingers, or by lowering the mass of the bridge members 41 and 43. However, increasing the number of fingers requires additional fabrication, such as micromachining, which adds cost to the device and increases the chance that a device will need to be rejected due to a fabrication error. Further, as more fingers are added, the more likely it becomes that the fingers will not be properly interdigitated and the device will not operate correctly (or at all). The second solution, decreasing the mass of the bridge members, can leave the bridge members with insufficient resistance to torsional bending, or allow then to deform unevenly, both of which can cause fingers 44 and 45 which are supported on the mass element 42 to bind with the static fingers 46 and 47. This can cause the device 40 to malfunction or to even be irreparably damaged.
What is needed then is a beam for supporting a mass element within a MEMS which achieves the benefits to be derived from similar prior art devices, but which avoids the shortcomings and detriments individually associated therewith.
The present invention provides for a microelectromechanical device, which includes a mass element, which is supported by one or more support beams. The support beam(s) comprise a first beam member and a second beam member, which in combination support the mass element free from a surrounding substrate. The beam members are connected to at least one, and preferably a plurality, of cross members, which connect the beam members into a resulting support beam. The resulting support beam is of relatively low mass and also provides for relatively high flexibility of the mass element in the intended direction of travel. The resulting support beam is also relatively stiff in a direction perpendicular to the intended direction or travel, and also provides for relatively high resistance to torsional deformation of the support beam. The mass element can be supported in a cantilevered manner by a single support beam in accordance with the present invention, or it can be supported by two support beams of the present invention in a bridge configuration.
In one embodiment the present invention provides for a microelectromechanical device defined within a substrate of a MEMS. The microelectromechanical device (xe2x80x9cMEMDxe2x80x9d) can be for example a microsensor or a microactuator. The MEMD includes a mass element defining an area of interest. The area of interest can support additional components used to give the microelectromechanical device its functionality. The device also includes a support beam supporting the mass element in spaced-apart relationship from the substrate. The support beam comprises a first beam member defined by a first fixed end connected to the substrate, and a first free end connected to the mass element. The support beam further comprises a second beam member defined by a second fixed end connected to the substrate, and a second free end connected to the mass element, the second beam member being spaced apart from the first beam member. Finally, the support beam includes a first cross member connecting the first beam member and the second beam member.
Preferably the first beam member, the second beam member, the first cross member and the mass element define a first void, while the first beam member, the second beam member, the first cross member and the substrate define a second void. That is, the cross member bifurcates a void which is formed by the two beam members, the mass element and the substrate. In one configuration the beam members are substantially parallel, and the first cross member is substantially perpendicular to the beam members. In another configuration the first cross member is positioned at an angle to the beam members. The first cross member can be connected to the beam members across their entire height, or across only a portion of the height of the beam members. In a preferred embodiment the microelectromechanical device further comprises a second cross member connecting the first beam member and the second beam member, and the second cross member is in spaced-apart relationship from the first cross member. More preferably, the microelectromechanical device comprises a plurality of cross members connecting the beam members, and each of the cross members are in spaced-apart relationship from one another. In one variation wherein a plurality of cross members are used one of the cross members intersects another of the first members. In yet another variation, rather than the cross member connecting the beam members across the entire height of the beam members, the cross member connects the beam members at the upper edge (and/or the lower edge) of the beam members.
A second embodiment of the invention provides for a microelectromechanical device, which is a bridge structure, such that the mass element is supported between two support beams. More specifically, the microelectromechanical device is defined within a substrate of MEMS, and the MEMD comprises a mass element defining an area of interest. The mass element is defined by a first side and an opposite second side. The device includes a bridge supporting the mass element in spaced-apart relationship from the substrate. The bridge comprises a first support beam and a second support beam. Each support beam comprises a first beam member and a second beam member. The first and second beam members are defined by respective first and second fixed ends connected to the substrate. The first and second beam members are further defined by respective first and second free ends, which are connected to the mass element at the respective first and second edges thereof. The first and second beam members of each support beam are spaced apart from one another. Each support beam also includes a first cross member connecting the first and second beam members of each support beam.
Preferably, the microelectromechanical device having a bridge structure is configured such that each support beam further comprises a plurality of cross members connecting first and second beam members of each support beam. The cross members in each support beam can be in spaced apart relationship from one another, or they can intersect one another (or they can be a combination of spaced-apart and intersecting cross members).
A third embodiment of the present invention provides for a method of forming a microelectromechanical device, which has a mass element. The method includes the steps of depositing a substrate layer and then forming the mass element by removing at least a portion of the substrate layer. The resulting mass element defines an area of interest free of contact with the substrate. The substrate can include a plurality of different layers formed using known depositional technologies, and portions of the substrate can be removed also using known technologies, such as photolithography, wet etching, dry etching, and micromachining using a focused ion beam such as an excimer laser. The method further includes forming a first and a second beam member by removing at least a portion of the substrate. The beam members are spaced-apart from one another, and each resulting beam member is defined by a first fixed end connected to the substrate, and a first free end connected to the mass element. A cross member connecting the first beam member and the second beam member is formed by removing at least a portion of the substrate. The steps of forming the mass element, the beam members and the cross member can all be performed at the same time, as for example in an etching process.
As mentioned above, the substrate can be deposited as plurality of layers. At least one of the plurality of layers can comprise a sacrificial oxide, and at least one of the plurality of layers can comprise a polysilicon. The step of removing part of the substrate to form the mass element, the beam members and the cross member can be performed by etching away at least a part of the sacrificial oxide to leave the mass element, the beam members, and the cross member, all of which are formed from the polysilicon. In one variation the cross member can comprise a boron diffusion in the substrate. The substrate surrounding the boron diffusion can be removed for example by etching the substrate with potassium hydroxide (KOH).
These and other aspects and embodiments of the present invention will now be described in detail with reference to the accompanying drawings, wherein: